Delaying involves sending a temporary 451 SMTP error code to the sending server when a message is received, along with sending this error code ASSP creates a Triplet and stores this. On the second delivery attempt if the Embargo Time set by the ASSP admin for the Triplet has been surpassed the message will be accepted and a Tuplet will be created and not delayed again for an Expiry Time set by the ASSP admin.
This method is effective because the majority of spammers do not retry to send their messages and so their messages are never accepted. This is also somewhat useful in stopping viruses that contain their own smtp engine as these are not sophisticated enough to handle message queuing .
How it works
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A simple explanation is as follows
1. A message is received by ASSP; the IP address of the sending host + senders domain are checked against a whitelist.
2. If the combination of the two pieces of information is found in the whitelist the message is allowed through.
3. If the combination is not found then a Triplet (the IP address of the sending host, sender, and recipient) is created and added to the delaying database and the sending MTA is informed that there has been an error by means of a 451 SMTP error code. The sending MTA should know that this is not a permanent error and should retry the message after a period of time.
4. When the message matching a Triplet is sent again after the Embargo Time, it will be accepted and the information on the IP address of the sending host + sender's domain is added to the whitelist as a Tuplet to prevent further delays for this combination.
5. The various time parameters involved are configurable for fine-tuning.
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